I don't really think this is a society problem but a pschyological behavior that doesn't have outside influence as the MAIN cause.
I don't try to analyze things(because I am kind of bad at it) but mosly because I don't like to think in depth about matters.
In this situation, I think that there is some Fruedianism<er, spelling> that can be applied. For the most part, he claimed our actions were instigated by sexual needs. I think that people tend to see healthy people as slim and tone. They see heavy people as unhealthy. But it's their minds' eye that makes those statements. There is no translation that arises in the subconscious for those observations.
For the most part, if someone is overweight, they might have something like diabetes. But even thin people can have health problems. If you ever notice, it is not just a young man that wants to talk to a beautiful teenage girl. She attracts the attention of everybody. Her friends want to be around her, little kids want to go places with her, family tends to pay more attention to her. And a handsome young man will not just get the attention of girls. People will see him as capable, heallthy. Able to do his job and take care of a family. I think that our society projects a glammourous image of how we SHOULD look, yes. But it is already established. If there was no media and TV, we would still like that body and try for that body. It makes us feel better and younger, healthier. Even if you are happy and you do not fit into that perfect body scenario, I don't know anyone who would prefer to be overweight as opposed to be tone and trim.
Why do people that are heavy get looked down upon? Well, not everyone shuns you if you have a weight problem, but on the same hand, an individual who is overweight or obese is perhaps seen with an eating problem. As someone with no self-control. As a depressed and angry person. the list goes on. And we know this is a stereotype that can not apply to most persons who need to lose weight.
The person who is overweight tends to be aware that he/she is overweight and then when talking to a person who is of a normal weight, that awareness just keeps festering. When talking to an overweight person, someone who does not have a weight problem tries to ignore that other person's difference, because THAT is something our society teaches us to do. Especially an american society. It teaches us to assimilate. So aside from snide mockings of teenage boys now and then, majority of people do not normally make comments if you have weight issues.
When you are heavy, overall people see you as an individual I think, and there is wasted potiential physically if you do not live up to the human expectation of having a nice body. And simply, if you do live up to that potiential, you are admired and admiration is the thing we have for movie stars and super athletes, on a different scale though.
Visually pleasing things makes us happy, but visually unpleasing things make us uncomfortable. That is why the entertainment industry is so big. Why pornography exsists. Why models are paid so much to wear a christian dior. why we flock to see movies when books have better stories.